Add support for printing multicol containers, and enable it.

third_party/WebKit/Source/core/layout/LayoutView.cpp

Index: third_party/WebKit/Source/core/layout/LayoutView.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutView.cpp b/third_party/WebKit/Source/core/layout/LayoutView.cpp
index 39e158b8f1344047fd86812ffcfe295aba01625c..8fb1d400fa8866f10b67abdda0202cbffe3eb474 100644
--- a/third_party/WebKit/Source/core/layout/LayoutView.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutView.cpp
@@ -38,6 +38,7 @@
 #include "core/layout/LayoutPart.h"
 #include "core/layout/LayoutQuote.h"
 #include "core/layout/LayoutScrollbarPart.h"
+#include "core/layout/ViewFragmentationContext.h"
 #include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/page/Page.h"
 #include "core/paint/PaintLayer.h"
@@ -249,6 +250,14 @@ void LayoutView::layout()
 
     m_pageLogicalHeightChanged = false;
 
+    if (m_pageLogicalHeight) {
+        if (!m_fragmentationContext)
+            m_fragmentationContext = adoptPtr(new ViewFragmentationContext(*this));
+    } else {
+        if (m_fragmentationContext)
+            m_fragmentationContext.clear();

[leviw_travelin_and_unemployed, 2015/12/08 19:00:55]

It seems a little weird to new a ViewFragmentationContext every time the view
gets height. We could avoid some thrashing if FragmentationContext had a concept
of being active.

I see we can set the pageLogicalHeight above:
if (!document().paginated())
        setPageLogicalHeight(0);

but then we may bail before getting to this code if !needsLayout(). Does this
code properly account for that case?

[mstensho (USE GERRIT), 2015/12/08 20:09:12]

Gets *page* height, not just any height. Right?

That's definitely not my intention. I only want to new it every time we enter
printing (i.e. get a page logical height). And I want to delete it when we leave
printing.
Sure, that would be easy enough to do (just add a flag, and have the descendant
multicol container ask "Do I have an enclosing ViewFragmentationContext? Am I
sure? Hey, you up there, are you active?"). But do we really need it?
I would imagine that switching from printing mode to normal mode will always
mark the view for layout, but yeah, better safe than sorry.

I'll move the code a bit further up, and piggy-back it on
shouldUsePrintingLayout() (only the root frame will normally paginate, so this
makes sense to check). I'll wait for more comments from you before doing
anything else.

+    }
+
     layoutContent();
 
     if (layoutOverflowRect() != oldLayoutOverflowRect) {